1. Field of the Invention
The present invention relates to laser printers in general, and in particular, to a laser printer with a laser diode array source, a spatial light modulator, a laser lenslet array, and a fly's eye integrator assembly.
2. Description of the Prior Art
In a typical laser printer, radiation from a laser is shaped, and imaged onto a film plane to produce a desired spot size. The spot, called a pixel, forms the smallest element of the image. The laser radiation is modulated to create the correct density of each spot, pixel by pixel. The laser spot is scanned in the line direction, and the media is moved in the page direction to create a two dimensional image.
In a printer system with a continuous wave (CW) gas or solid state laser, an external modulator, such as an acousto-optical device, is used to input the image data into the optical beam. For systems with semi-conductor diode lasers, the laser radiation is typically modulated directly by varying the current input to the laser. For printers using high sensitivity media such as silver halide, high throughput is obtained by scanning the laser beam in the line direction with a polygonal mirror or a galvanometer. These printers are called "flying spot" printers.
For a low sensitivity media printer, such as a laser thermal printer, higher power laser sources and slow line and page scan speeds are used to meet the high exposure requirements, which are typically 0.2-0.5 joules/cm.sup.2. One way to achieve this type of scan is to configure the printer like a "lathe", where the page scan is obtained by rotating a drum which holds the film, and line scan, by translating the laser in a direction parallel to the axis of rotation of the drum.
To achieve this high optical power throughput in a small package with a relatively low cost, many discrete lasers are ganged together to form multiple spots on the film plane. Multiple spots, or pixels, written simultaneously, increase the throughput, and multiple laser sources provide the required optical energy. There are several approaches for bringing a multitude of laser sources together in a laser printer, including a system wherein the laser sources are separately coupled to optical fibers, which are then mounted together to form a linear array of sources. Such a system is described in U.S. Pat. No. 4,911,526.
Another approach is to utilize a monolithic array of laser sources, and then image the elements of the laser array directly onto the light sensitive media to produce multiple spots. Power to each element of the laser array is individually modulated to obtain pixel densities. Such a system, described by U.S. Pat. No. 4,804,975, is potentially of lower cost and higher efficiency compared to systems which couple the lasers to optical fibers. However, such monolithic laser diode arrays have their own disadvantages. When the individual lasing elements are imaged directly to the media, failure of even one element in the array introduces a pattern error. Also, the electronics to modulate high current inputs to the diodes at high speeds are expensive and difficult to manufacture. Furthermore, the system is also sensitive to image artifacts caused by thermal and electrical crosstalk effects within the diode laser array package.
One approach to improving a system with a monolithic diode array source is to split each lasing element into an array of subarray laser sources. Systems employing such lasers are described in U.S. patent application Ser. No. 07/986,207, filed Dec. 7, 1992, U.S. Pat. No. 5,795,153 and Ser. No. 08/283,003, filed Jul. 29, 1994 5,613,245 to the same assignee as the present invention. Each writing pixel is assembled from the combined light of all the lasing elements of a given subarray, and each of the subarrays are directly and individually modulated to provide the image data input. This approach helps to reduce the sensitivity to thermal crosstalk and as well desensitizing the system to failure of lasing elements within a subarray.
Another approach to improving a system with a monolithic diode array sources is to combine the light from each lasing element to flood illuminate a spatial light modulator. The elements of the modulator break up the light into image elements, and each element of the modulator is subsequently imaged onto the media plane to form the desired array of printing spots. Systems employing this approach are described in U.S. Pat. Nos. 5,517,359 and 5,521,748. These systems improve upon the prior art designs by providing indirect light modulation means, so that the laser diode array source operates at full power, and serves only as a light source. The systems described in these patents have a disadvantage in that the illumination provided to the modulator plane may be substantially non-uniform. In both cases, the emitting elements are imaged directly onto the modulator at a high magnification. As the array direction light emission profiles have both macro- and micro- nonuniformities, the resulting modulator illumination may be significantly non-uniform. U.S. Pat. No. 5,517,359 includes a mirror system which partially compensates for these problems by substantially removing the macro-nonuniformities, but at the cost of some reduced brightness due to the increased angular spread of illumination to the modulator. Also, this uniformization method only works well when the light profile across the emitting elements already has large areas which are substantially uniform.
There are numerous methods available for improving the illumination uniformity of optical systems, such as with diusers and integrating cavities. However, most of these methods substantially reduce the brightness of the light conducted through the entire optical system. The optical systems designed for photolithographic printing in the manufacture of a semiconductor element, such as for an IC chip, have made extensive use of integrators which substantially preserve the brightness while providing uniform illumination. In particular, fly's eye lenslet array systems have been used in many systems, such as that described by U.S. Pat. No. 4,497,015, Konno et al., which utilizes an arc lamp source, the light output of which is carefully homogenized before it arrives at the mask plane. Another patent, U.S. Pat. No. 4,939,630, describes a similar photolithographic illumination system, which utilizes a laser, such as an excimer or YAG laser, as the source. These light sources, the arc lamp, excimer laser, and YAG laser, are large, spatially continuous sources, which emit beams of substantial size. By comparison, a laser diode array is a segmented source, which consists of a series of small, distinct emitting sources, substantially separated from each other spatially.